arXiv2026
The rapid evolution of cyberattacks continues to drive the emergence of unknown (zero-day) threats, posing significant challenges for network intrusion detection systems in Internet of Things (IoT) networks. Existing machine learning and deep learning approaches typically rely on large labeled datasets, payload inspection, or closed-set classification, limiting their effectiveness under data scarcity, encrypted traffic, and distribution shifts. To address these limitations, we propose SiamXBERT, a data-efficient, open-set Siamese meta-learning framework built on a pretrained language model for unknown-attack detection. The framework constructs a dual-modality feature representation by integrating flow-level and packet-level information, enabling richer behavioral modeling while remaining compatible with encrypted traffic. The model learns similarity patterns for known benign and attack classes using 100 labeled samples per class. It flags traffic that does not sufficiently match any known class as potentially unknown, using a threshold set without unknown-attack data. We evaluate SiamXBERT on two IoT intrusion detection datasets against five state-of-the-art detection methods. When training and testing are performed on the same dataset at this labeling budget, SiamXBERT attains the highest unknown F1-score, exceeding the strongest competing method on each dataset by 84.8% and 37.4%; the improvement is statistically significant against all five methods on one dataset and against four of the five on the other. When a model trained on one dataset is tested on another, SiamXBERT transfers effectively in one direction but degrades in the reverse, reflecting the rejection behavior learned from the source traffic rather than the choice of threshold. These results highlight the practicality of SiamXBERT for data-efficient unknown attack detection in real-world IoT environments.